1. Field of the Invention
The present general inventive concept relates to an image forming apparatus and an image forming method thereof, and more particularly, to an image forming apparatus which improves a printing quality, and an image forming method thereof.
2. Description of the Related Art
An electrophotograhic image forming apparatus forms an image on a printing medium through charging, exposing, developing, transferring and fixing processes. The electrophotograhic image forming apparatus includes a laser printer, a photocopier, a multifunction printer, etc.
As illustrated in FIGS. 1A and 1B, a conventional electrophotograhic image forming apparatus 1 includes a supplying roller 10, a developing roller 20, a charging roller 30, a photosensitive drum 40, a transfer roller 50, and a moving roller 60. The electrophotograhic image forming apparatus 1 further includes an exposing unit (not illustrated) which exposes a surface of the photosensitive drum 40 to light L.
The charging roller 30 receives approximately −1,400 Vdc from a high voltage power supply (HVPS) which is provided in the image forming apparatus 1 and supplies a high voltage, and contacts and charges the photosensitive drum 40 to provide a surface potential of about −800 Vdc. The surface of the photosensitive drum 40 is exposed to light corresponding to image information by the exposing unit, thereby forming an electrostatic latent image thereon. An exposing region of the latent image has an electric potential of approximately −50 Vdc, which is higher than a non-exposing region of −800 Vdc.
The supplying roller 10 supplies a toner accommodated in a toner accommodator (not illustrated) to the developing roller 20, and frictionally charges the toner with a negative charge. A thickness of the toner supplied to the developing roller 20 is controlled by a regulating blade 3. The developing roller 20 develops the electrostatic latent image formed on the surface of the photosensitive drum 40 with the toner. Then, a visible toner image including the toner is formed on the surface of the photosensitive drum 40.
The HVPS supplies a high voltage, i.e., approximate +1,200 Vdc and having an opposite polarity to the negative charge of the toner, to the transfer roller 50. The visible toner image is transferred to a printing medium P1 which passes a transfer region A formed between the photosensitive drum 40 and the transfer roller 50. The toner which remains in the surface of the photosensitive drum 40 is stacked in a cleaning film 5. A part of the remaining toner passes the cleaning film 5 and is retrieved to the developing roller 20 by a nip between the photosensitive drum 40 and the developing roller 20.
The visible toner image which is transferred to the printing medium P1 is fixed thereto by heat and pressure, thereby completing a printing process.
About 2,000 Vdc potential difference exists between the photosensitive drum 40 and the transfer roller 50. Thus, if the printing medium P1 passes a transfer region A, the printing medium P1 has an induced charge. As illustrated in FIG. 1B, the printing medium P1 is charged with a positive charge by the transfer roller 50. The moment the printing medium P1 is separated from the photosensitive drum 40, the positive charge of the printing medium P1 is introduced to a rear end contact surface C of the photosensitive drum 40 contacting an end part of the printing medium P1. As illustrated in FIG. 1C, the rear end contact surface C has a higher electric potential than the nearby surface potential of −800 Vdc in a lengthwise direction. The potential difference between the rear end contact surface C and the photosensitive drum 40 having −800V surface potential is continuously maintained to cause poor picture quality.
As the photosensitive drum 40 rotates, the remaining toner T having the negative charge and being stacked in the cleaning film 5 contacts the rear end contact surface C having a relatively smaller electric repulsion than the −800 Vdc surface potential of the photosensitive drum 40. Then, a line image appears in a subsequent printing medium P2 in a transverse direction with respect to a paper-feeding direction, thereby lowering printing quality.